[PATCH] defxx: Remove an incorrectly inverted preprocessor conditional

[PATCH] defxx: Remove an incorrectly inverted preprocessor conditional

[Maciej W. Rozycki]

The RX handler of the driver has two paths switched between, depending on 
the size of the frame received, as determined by SKBUFF_RX_COPYBREAK.  

When a small frame is received, a new skb allocated has data space large 
enough to hold the incoming frame only, and data is copied there from the 
original skb whose buffer is returned to the DMA RX ring; in that case 
`rx_in_place' is 0.  When a large frame is received, a new skb allocated 
has data space large enough to hold the largest frame possible, including 
the overhead for alignment, the receive status and padding, over 4.5kiB 
overall, and its buffer is placed on the DMA RX ring while the original 
buffer is passed up to the network stack avoiding the need to copy data; 
in that case `rx_in_place' is 1.

However the latter scenario is only possible when dynamic buffers are 
used, as determined by DYNAMIC_BUFFERS, because otherwise the buffers used 
for the DMA RX ring are fixed at the time the interface is brought up.

That leads to an observation that the preprocessor conditional around the 
`rx_in_place' check is inverted, the check only really matters when 
dynamic buffers are in use.  It has gone unnoticed for many years since 
support for using dynamic buffers on the DMA RX ring was introduced in 
2.1.40 -- because the only problem that results is in the case where 
`rx_in_place' is 1 frame data received is unnecessarily copied to the 
newly-allocated buffer, before the buffer placed on the the DMA receive RX 
and its contents ignored.  Therefore the only symptom is some performance 
loss.

Rather than flipping the condition though I decided to discard the 
conditional altogether -- in the case of static buffers `rx_in_place' is 
always 0 so GCC will optimise the C conditional away instead.

Tested on a few DEFPA and DEFTA boards successfully using both small and 
large frames, both with DYNAMIC_BUFFERS defined and with the macro 
undefined.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
---
linux-defxx-skb-rx-copy-fix.patch
Index: linux-20140623-4maxp64/drivers/net/fddi/defxx.c
===================================================================
--- linux-20140623-4maxp64.orig/drivers/net/fddi/defxx.c
+++ linux-20140623-4maxp64/drivers/net/fddi/defxx.c
@@ -3074,10 +3074,7 @@ static void dfx_rcv_queue_process(
 					break;
 					}
 				else {
-#ifndef DYNAMIC_BUFFERS
-					if (! rx_in_place)
-#endif
-					{
+					if (!rx_in_place) {
 						/* Receive buffer allocated, pass receive packet up */
 
 						skb_copy_to_linear_data(skb,

Re: [PATCH] defxx: Remove an incorrectly inverted preprocessor conditional

[David Miller]

From: "Maciej W. Rozycki" <macro@linux-mips.org>
Date: Sun, 29 Jun 2014 01:45:53 +0100 (BST)

> The RX handler of the driver has two paths switched between, depending on 
> the size of the frame received, as determined by SKBUFF_RX_COPYBREAK.  
> 
> When a small frame is received, a new skb allocated has data space large 
> enough to hold the incoming frame only, and data is copied there from the 
> original skb whose buffer is returned to the DMA RX ring; in that case 
> `rx_in_place' is 0.  When a large frame is received, a new skb allocated 
> has data space large enough to hold the largest frame possible, including 
> the overhead for alignment, the receive status and padding, over 4.5kiB 
> overall, and its buffer is placed on the DMA RX ring while the original 
> buffer is passed up to the network stack avoiding the need to copy data; 
> in that case `rx_in_place' is 1.
> 
> However the latter scenario is only possible when dynamic buffers are 
> used, as determined by DYNAMIC_BUFFERS, because otherwise the buffers used 
> for the DMA RX ring are fixed at the time the interface is brought up.
> 
> That leads to an observation that the preprocessor conditional around the 
> `rx_in_place' check is inverted, the check only really matters when 
> dynamic buffers are in use.  It has gone unnoticed for many years since 
> support for using dynamic buffers on the DMA RX ring was introduced in 
> 2.1.40 -- because the only problem that results is in the case where 
> `rx_in_place' is 1 frame data received is unnecessarily copied to the 
> newly-allocated buffer, before the buffer placed on the the DMA receive RX 
> and its contents ignored.  Therefore the only symptom is some performance 
> loss.
> 
> Rather than flipping the condition though I decided to discard the 
> conditional altogether -- in the case of static buffers `rx_in_place' is 
> always 0 so GCC will optimise the C conditional away instead.
> 
> Tested on a few DEFPA and DEFTA boards successfully using both small and 
> large frames, both with DYNAMIC_BUFFERS defined and with the macro 
> undefined.
> 
> Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>

Applied, thanks.